There are two types of electromagnetic fields: Constant and Alternating.

Constant fields

Here, the direction does not change. Examples include the Earth’s natural electric and magnetic fields, the magnetic fields used to run subways and streetcars, and the fields used in magnetic resonance imaging (MRI).   

Alternating fields

Here, the direction does change. The number of periods per second is equal to the frequency, and the unit of frequency is Hertz (Hz).   

Low-frequency fields (LF)

Alternating fields up to 30 kHz

HF, RF and microwave fields

Alternating fields from 30 kHz to 300 GHz